1. Field Of The Invention
The present invention relates to a tuned engine mounting system for a jet aircraft, and more particularly to an engine mounting system in which an aft engine mount is provided with spring beams for damping pivotal movement of the engine.
2. Background Information
It is a design goal that when an aircraft wing is subjected to a momentary force in flight which causes the wing to oscillate between a bent state and an unbent state, that in the absence of this force, the oscillations will damp out and the wing will return to a steady, unbended state. On the other hand, "wing flutter" refers to a phenomenon in which the wing oscillations between the bent state and the unbent state do not damp out. Rather, the amplitude of these oscillations either remains constant or increases over time.
Wing flutter is an aeroelastic instability produced by the coalescing and proper phasing of two or more structural vibration modes of an aircraft in flight. A flutter mode usually involves both bending and torsion-types of motion in which the torsional motion extracts energy from the airstream and drives the bending mode to increasingly higher amplitudes. In other cases, these oscillations are lightly damped, but stable, within the operating speed envelope of the aircraft and can cause a reduction in riding comfort of the aircraft.
The location of the engine nacelle relative to the wing, the mass properties of the engine, and the stiffness of the strut which attaches the nacelle to the wing are factors which influence the flutter characteristics of the wing. More specifically, the natural frequency of the nacelle and the manner of strut installation can influence the mode and air speed at which the wing oscillations become unstable (flutter).
Conventionally, in order to avoid wing flutter, the natural frequency of the nacelles and nacelle struts are restricted within a narrow range. For example, in earlier models of the Boeing 747 aircraft, the outboard engine nacelles are permitted to oscillate at a natural frequency of about 2 cycles per second in a lateral direction. If the outboard engine nacelle lateral frequencies are significantly above or below 2 cycles per second, then wing flutter can result at an unacceptably low air speed.
However, in some newer aircraft which feature stronger but less stiff lifting surfaces, flutter can occur at air speeds below that required by government regulations. In this case, the avoidance of wing flutter requires the unsatisfactory solution of reducing the maximum operating speeds of the aircraft.
U.S. Pat. No. 4,917,331 discloses a method for preventing wing flutter in an aircraft, wherein the lateral natural frequencies of the left and right engines are sufficiently different so that when subjected to a time varying disturbance in flight, the flutter speed of the aircraft is increased. In this design, spring beams are attached at the interface between the pylon structure and the wing for damping movement of the engine. The spring beams may be tuned to provide the desired lateral frequency.
FIG. 1 shows a prior art engine and wing structure, wherein a wing 10 supports a pylon structure 12, which supports the engine 14. The pylon structure 12 is attached to the wing 10 at the upper link interface 16 and mid-spar fittings 18. The spring beam structure described in U.S. Pat. No. 4,917,331, referenced above, would be attached at the interface between the wing 10 and the pylon structure 12, such as in the areas of the upper link interface 16 or mid-spar fittings 18 shown in FIG. 1. The spring beam attachment structure would replace such attachment devices 16,18. Because the spring beams are spaced substantially from the center line 20 of the engine (the center of mass), the placement of such spring beams is limited, and the size of the entire spring beam attachment structure is significant as a result of strength requirements. Accordingly, the spring beam attachment structure generally defines the placement of the engine 14. In other words, as a result of the strength limitations of the attachment, the engine 14 may not be positioned where desired to optimize efficiency.